#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	int n,i;
	double a,b,t;
	double x=2;
	double y=1;
	double sum=x/y;
	cin>>n;
	a=x;
	b=y;
	for(i=1;i<n;i++)
	{
		t=a;
		a=a+b;
		b=t;
		sum+=1.0*a/b;
	}
	printf("%.4f",sum);
	return 0;
}
